Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixel cells or pixels as an imaging sensor, each cell including a photosensor, which may be a photogate, photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor, typically a source follower transistor, which is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference. A row select transistor may also be employed to gate the pixel output.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) storage of charge in the storage region: (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region.
CMOS imaging devices of the type discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, assigned to Micron Technology, Inc.
Ideally, the digital images created by a CMOS imaging device are exact duplications of the light image projected upon the device's pixel array. That is, for a flat-field image, all of the imaging pixel signals should have the same signal value. However, various noise sources can affect individual pixel outputs and thus distort the resulting digital image. As CMOS pixel arrays increase in size to obtain higher resolution, the physical non-uniformity of the arrays becomes more prominent. One issue is the signal variation between rows that can result in vertical shading.
One known solution is to include in the array dark reference pixels that are light-shielded to determine the noise in each row. Because the dark reference pixels are light shielded, and therefore optically black, all voltage that is output by the pixels is generated by noise and not by light. The noise signals received from the dark reference pixels for a given row are averaged and used in the digital domain to remove noise from the image signals output from the row.
The above solution to row noise may be adversely affected by what is known as “hot” pixels. A hot pixel is a pixel that appears bright when it is supposed to be completely dark black. Hot pixels are typically caused by process defects such as e.g., silicon defects, metallic contamination, stress, etc. When one or more of the reference pixels contain hot pixels, the row noise correction can introduce a correction offset that is not truly representative of the row noise. That is, the values of the hot reference pixels adversely affect the row noise compensation average discussed above: this improper average will cause an incorrect compensation offset to be applied to the image pixels in that row, causing the entire row of image pixels to have inaccurate values. Methods for filtering hot pixels exist, but current methods involve filtering hot pixels in the digital domain (i.e., after pixel signals have been converted from analog to digital) or involve using dark reference pixels with a different electronic structure than the imaging pixels.
Noise correction and or hot pixel filtering in the digital domain suffers from a lack of accuracy because other noise is introduced between the analog pixel and the digital processing circuits that make row noise correction and/or hot pixel filtering less accurate. To address the above limitation of the digital row noise correction, a method and apparatus for row noise correction and hot pixel filtering in the analog domain (i.e., prior to digital conversion) is provided. Furthermore, compared to its digital domain counterpart, analog row noise correction has the advantage of smaller die size, greater accuracy and faster readout speed.